Recent progress in power electronics makes it possible to combine a conventional mechanical circuit breaker with a semiconductor switch device into a so-called hybrid circuit breaker for interrupting a fault current in an electrical circuit line. Such a hybrid circuit breaker comprises a mechanical circuit breaker for interrupting the line upon detecting the fault current and comprising a movable contact and a stationary contact and a semiconductor device, a solid-state part, arranged in parallel with the mechanical circuit breaker to conduct the fault current when the mechanical circuit breaker is disconnected. Depending on device ratings, the semiconductor device may include a set of serial or parallel connected controllable semiconductor units, for example thyristors, GTOs (Gate Turn-Off thyristor), IGBTs (insulated gate bipolar transistor) or IGCTs (Integrated Gate Commutated Thyristor).
Under normal working conditions, the semiconductor units of the solid-state part are turned-off and the contacts of the mechanical breaker are in a closed position and current is conducted through the mechanical circuit breaker that has low on-state losses. When activated, for interrupting the current, the semiconductor units have to be fired first in order to provide a parallel branch for a current commutation process. The mechanical circuit breaker is disconnected by opening the contacts, leading to an arc voltage which is responsible for the commutation of the current to the parallel branch. Since the air gap between the contacts is not able to block the full voltage, the semiconductors must conduct the current for a certain amount of time. Once the contacts of the mechanical circuit breaker are locked sufficiently and completely disconnected, this holding-off interval, also called conduction time, is elapsed and the semiconductor units are turned off. Following turn-off of the semiconductor units, the stored energy in the loop inductance may be absorbed by the overvoltage protection element, such as a varistor.
Examples of such hybrid circuit breakers are shown in FIG. 1a and FIG. 1b. A U.S. Pat. No. 6,760,202 B1 discloses a hybrid circuit breaker comprising a mechanical breaker and a solid-state circuit breaker connected in parallel with the mechanical breaker. The solid-state part of the hybrid circuit breaker includes a diode bridge for making the breaker work for both directions of a current, at least one thyristor of the type IGCT for contacting a fault current when the mechanical breaker is open and a MOV (Metal Oxide Varistor) connected in parallel with the IGCT and for limiting voltages across the devices when the IGCT is opening and for dissipating the inductive energy of the main circuit line.
It takes a mechanical delay for the mechanical breaker to start to open upon detecting a fault current, which consequently influences a total reaction time for fault clearance. To achieve high quality of safety and reliability, it is desired that a hybrid circuit breaker reacts to a current fault as fast as possible and, meanwhile keeps the conduction time of the solid-state devices as low as possible so that no overheating is generated to lead failures of the devices.